1. Field of the Invention
The present invention relates generally to an optical transmission system for radio access and a high frequency optical transmitter, and more particularly, to an optical transmission system for connecting, in radio access for coupling a center station and a plurality of subscriber terminals with a radio signal (a high frequency radio signal in a microwave band, a millimeter wave band, or the like), the center station and a radio base station by an optical fiber, and a high frequency optical transmitter used for the system.
2. Description of the Background Art
FIG. 17 illustrates an example of the configuration of a conventional optical transmission system used for radio access for connecting a center station and a subscriber terminal through a radio base station for transmitting and receiving a radio signal.
The conventional optical transmission system shown in FIG. 17 is constructed by respectively connecting a center station 600 to a plurality of radio base stations 701 to 70n (n is an integer of not less than two; which is the same in the following present specification) through a plurality of downstream (from the center station to the radio base stations) optical fibers 801 to 80n. The center station 600 includes a plurality of electrical-optical converters 611 to 61n respectively corresponding to the plurality of radio base stations 701 to 70n. The radio base stations 701 to 70n respectively include optical-electrical converters 711 to 71n, modulators 721 to 72n, frequency converters 731 to 73n, local oscillation signal sources 741 to 74n, and antennas 751 to 75n. The operation of the conventional optical transmission system will be described.
In the center station 600, information to be transmitted to the subscriber terminal through the radio base station 70k is inputted in the form of a baseband signal to an input terminal 6k (k=1 to n; which is the same in the following present specification). The electrical-optical converter 61k converts the baseband signal inputted from the input terminal 6k into an optical signal. The optical signal outputted from the electrical-optical converter 61k is transmitted to the radio base station 70k from the center station 600 through the downstream optical fiber 80k. 
In the radio base station 70k, the optical signal transmitted from the center station 600 is inputted to the optical-electrical converter 71k. The optical-electrical converter 71k converts the inputted optical signal into an electric signal. The modulator 72k converts the electric signal obtained by the conversion into a signal having an intermediate frequency (an IF signal). The local oscillation signal source 74k outputs a local oscillation signal having a predetermined frequency. The frequency converter 73k receives the IF signal and the local oscillation signal, and converts the IF signal into a signal having a radio frequency (an RF signal) using the local oscillation signal. The RF signal is released to space through the antenna 75k. 
FIG. 18 shows, in a case where the center station 600 and the plurality of radio base stations 701 to 707 are connected to each other, the concept of a service area covered by each of the radio base stations. Areas 901 to 907 shown in FIG. 18 represent service areas respectively covered by the radio base stations 701 to 707.
Light signals respectively having different information are respectively transmitted to the radio base stations 701 to 707 from the center station 600 through the downstream optical fibers 801 to 807. In order to avoid interference between the adjacent service areas, the plurality of radio base stations 701 to 707 respectively change the frequencies of local oscillation signals outputted from the local oscillation signal sources 741 to 747 provided therein and convert IF signals into RF signals having different frequencies (fd1 to fd7), to perform radio transmission to the subscriber terminals. With respect to the radio base stations respectively covering the service areas which are not adjacent to each other (which correspond to the radio base stations 701, 705, and 706 in the example of FIG. 18), the same radio frequency (fd1=fd5=fd6) may be set.
However, when the different information are optically transmitted from the center station 600 to the plurality of radio base stations 701 to 70n and then to many subscriber terminals, as shown in FIGS. 17 and 18, various problems arise as follows.
The first problem is that the electrical-optical converters 611 to 61n, whose number (=n) corresponds to the number of the radio base stations 701 to 70n, are required in the center station 600.
The second problem is that the expensive frequency converters 731 to 73n for frequency-converting the IF signals into the RF signals are respectively required in the plurality of radio base stations 701 to 70n. 
The third problem is that when information to be transmitted to a lot of subscriber terminals are transmitted upon being time-division multiplexed, a multiplexer is required in the center station 600. In this case, separators are respectively required in the radio base stations 701 to 70n, and high-speed modulation processing is required for each of the modulators 721 to 72n. 
The fourth problem is that when the capacity of the one radio base station 70k (the amount of information which can be transmitted from the antenna 75k to the subscriber terminal) is increased for a new subscriber terminal, each of components other than the antenna 75k must be additionally installed in the radio base station 70k, and a multiplexer for multiplexing the RF signals is also required. Particularly when the position of the subscriber terminal to be added is the position where a substantially unobstructed line-of-sight propagation path cannot be ensured from the existing radio base station 70k, the components shown in FIG. 17 must be all newly installed such that the line-of-sight propagation path can be ensured.
The fifth problem is that the frequencies of the local oscillation signals outputted by the local oscillation signal source 74k in the radio base station 70k must be made to differ in order to avoid the interference between the adjacent service areas. Therefore, different components or different adjustments (if with the same components) are required for each radio base station 70k. 
FIG. 19 illustrates the configuration of another conventional optical transmission system in which the configuration of each of radio base stations 701 to 70n is simplified.
The conventional optical transmission system shown in FIG. 19 is constructed by respectively connecting a center station 600 and the plurality of radio base stations 701 to 70n through a plurality of downstream optical fibers 801 to 80n. The center station 600 respectively include modulators 621 to 62n, frequency converters 631 to 63n, local oscillation signal sources 641 to 64n, external modulators 651 to 65n, and light sources 661 to 66n so as to correspond to the radio base stations 701 to 70n. The radio base stations 701 to 70n respectively include optical-electrical converters 711 to 71n and antennas 751 to 75n. The operation of the conventional optical transmission system will be described.
In the center station 600, information to be transmitted to the radio base station 70k is inputted in the form of a baseband signal to an input terminal 6k. The modulator 62k modulates the baseband signal inputted from the input terminal 6k to an IF signal. The local oscillation signal source 64k outputs a local oscillation signal having a predetermined frequency. The frequency converter 63k receives the IF signal obtained by the modulation in the modulator 62k and the local oscillation signal outputted from the local oscillation signal source 64k, and frequency-converts the IF signal into an RF signal using the local oscillation signal. The light source 66k generates an optical signal having a predetermined wavelength. The external modulator 65k receives the RF signal obtained by the conversion in the frequency converter 63k and the optical signal outputted from the light source 66k, and intensity-modulates the optical signal using the RF signal. The intensity-modulated optical signal is transmitted to the radio base station 70k through the downstream optical fiber 80k. 
The optical signal transmitted from the center station 600 is inputted to the radio base station 70k upon propagating through the downstream optical fiber 80k. 
In the radio base station 70k, the optical-electrical converter 71k converts the inputted optical signal into an electric signal, to output an RF signal. The outputted RF signal is released into space from the antenna portion 75k to the subscriber terminal as a radio signal.
In the conventional optical transmission system, therefore, the IF signal is frequency-converted into the RF signal in the center station 600. Accordingly, the radio base stations 701 to 70n respectively require only the optical-electrical converters 711 to 71n in addition to the antenna portions 751 to 75n. Therefore, the conventional light transmission system has the effect of miniaturizing each of the radio base stations 701 to 70n. 
In the configuration of the other conventional optical transmission system shown in FIG. 19, however, the frequency converters 631 to 63n, the external modulators 651 to 65n, and the optical-electrical converters 711 to 71n must be high frequency devices (active devices) respectively operating in radio frequency bands. Such high-frequency devices are generally expensive. In such a configuration that the center station 600 manages the plurality of radio base stations 701 to 70n as in the other conventional optical transmission system, therefore, n expensive devices are required, so that the entire system becomes very expensive.